ELEMENTARY CHEMICAL PHYSIOLOGY 221 



similar treatment, which is the variety of pentose present in the nucleic acid 

 of animal cells. Pentose sometimes occurs in the urine the condition being 

 called pentosuria the variety being racemic arabinose (inactive optically). 

 From what source this is derived is difficult to determine, for it is independent 

 of the pentoses in the food, and its structure is different from that found 

 present in the tissues (see p. 325). 



CHAPTER VII 

 FATS 



Fats may generally be recognised by their various physical pro- 

 perties. They are insoluble in water, many are soluble with diffi- 

 culty in cold alcohol, they are freely soluble in ether, benzene, 

 chloroform, petrol, carbon disulphide, carbon tetrachloride, etc. 

 Liquid fats and fats of low melting-point give persistent " greasy " 

 (translucent) marks on paper. (The so-called " essential oils " 

 also give such marks but they do not persist they evaporate.) 

 Two or three reagents such as Osmic acid and Sudan III give 

 characteristic colour reactions. These substances are used as stains 

 in Histology for the detection of fatty matter in sections. 



Fats and Fatty Acids 



Neutral Fats are the ethereal salts of the fatty acids with the tri- 

 atomic alcohol glycerol, and have therefore the general formula : 



CH 2 -O -CO -X 



I 

 CH - - CO - X 



1 

 CH 2 - O - CO - X. 



They are named according to the fatty acid they contain, thus : 

 stearin, olein. The fatty acids are monobasic organic acids, con- 

 taining one carboxylic group (COOH) attached to a hydro- 

 carbon radicle. They belong to two classes, the saturated and 

 the unsaturated. The saturated acids have the general formula 

 C w H 2rt+1 .COOH. Those commonly occurring in fats are stearic 

 acid, in which n = 17, and palmitic acid, in which n = 15. 

 Thus the formula for stearic acid is CH 3 . (CH 2 ) 16 . COOH. 



The unsaturated acids contain relatively less hydrogen in the 

 hydrocarbon chain attached to the carboxylic group. This is due 

 to the fact that there are one or more double bonds (unsaturated) 

 between the carbon atoms of the chain. Thus oleic acid, the 

 commonly occurring unsaturated acid of fats, has the formula : 

 CH 3 . (CH 2 ) 7 . CH = CH(CH 2 ) 7 . COOH, and belongs to the series 

 QJlzn-i . COOH. Other unsaturated acids, containing two, or even 

 more, double bonds occur in the fat of the liver, heart and kidney. 

 The unsaturated nature of these acids is shown by their combining 

 directly with a halogen, thus becoming saturated. 



